553

The Canadian Mineralo gist Vol.32, pp. 553-561 (1994)

THESTRUCTURE AND PHYSICOCHEMICALCHARACTERISTICS OFA SYNTHETICPHASE COMPOSITIONALLY INTERMEDIATE BETWEENLIEBIGITE AND ANDERSONITE

RENAI.ID VOCHTEN, LAURENT VAN HAVERBEKE ANDKAREL VAN SPRINGEL

Iztboratortumvoor chemischeen fysische mineralagie, IJniversiteit Antwerpen (RUCA), MiddelheimlaanI, 8-2020 Antwerpm, Belgium

NORBERT BLATON AND OSWALD M. PEETERS

Laboratoriumvoor analytischechemie en medicbnlefysicochemie, Instituut voor FarmaceutischeWetenschnppen Kathnlieke Universiteitlzuven, VanEvenstraat 4, 8-3000 Leuven,Belgium

ABSIRACT

During the s1'nthesisof and andersonite,starting from tetrasodiumuranylcarbonate and ions, an unknown stableand crystalline intermediatephase is formed at a Ca2*/Na*ratio lying betweenthose necessary to form the above-cited compounds.Chemical analysis of this yellow-green intermediatephase results in the formula: Ca1.rNa6.6r[UO2(COr)3].rH2O ("r = 5.38). The scanningelectron micrographs show foliated lathlike .From single-crystatAata tni cryitats arb found to be orthorhombic,a 18.150(3),b 16.866(6),c 18.436(3)A, spacegroup Pnnm. The basicstrucrural units consisrof three UO2(CO3)3units. Each U atom is [8]-coordinated,resulting in a distorted hexagonal-bipy'ramidalpolyhedron. A chemical analysisindicates that the structureis short of positive charges,which could be balancedby the presenceof oxonium ions. Resultsof the -structureanalysis support this proposal,which ensuresa tight fit of the water moleculethat is not attachedto either Nar or Ca2+ions. The powder-diffractionspectrum calculated from the single-crystalstructffe corresponds very well with tle measuredone. The fluorescenceand phosphorescencespectra are given, as well as information on the thermaldecomposition, as revealed by thermogravimetricand differential scanningcalorimetric analyses.

Keywords:liebigite, andersonite,synthetic phase, ,physicochemical properties.

SoMraans

Au cours de nos synthbsesde la liebigite et de l'andersonite,i partir de I'uranylcarbonatede tetrasodiumet d'ions de calcium commemat6riaux de d6part,nous avonsd6couvert une phasemdconnue, stable et cristalline,ayant un rapporr ga2+/Na*entre ceux qui s'avdrentn6cessaires pour former les deux autrescompos6s. Une analysechimiqui de cene phase interm6diairejaune-vert mdne i la formuleCar.*Nan.ur[UO2(CO3)3].xH2O (.r= 5.38). D'aprbs les photospriies au microscope 6lectroniqueb balayage,il s'agit de cristauxen plaquettesayart une sructure en feuillets. Les donnfesobienues par diffraction X sur cristalunique montrent que cettephase est orthorhombique,a 18.150(3),b 16.866(6),c 18.436(3)A, groupespatial Pnnm. Les 616mentsstructuraux fondamentaux comprennent rois unit6s I stoechiom6trieUor(COr).. Chaqueatome d' possbdeune coordinencehuit et loge dans un polybdre hexagonalbipyrarnidal difformi. Uni anatysechimique montre que la structureaccuse un d6ficit de chargespositives, qui pounait 0tre satisfait par la prdsenced'ions oxonium. Les r6sultatsde l'6bauche de la structurecristalline confiflnent cette hypothdse,qui assureun adaptationserr6e de la mol6cule d'eau non rattachdeau Nat ou au Ca2*.Le spectrede diffraction X (m6thodedes poudres) calcul6 i partir desrdsultats obtenus sur cristal unique concordetrbs bien avec le spectremesur6. Nous d&rivons aussiles spectresde fluorescenceet de phospho- rescence,de mdmeque les r6sultatsde la d6compositionthermique par thermogravim6trieet par calorim6triediffdrentielle en modebalayage.

(Traduit par la R6daction)

Mots-cl6s:liebigite, andersonite,phase synth6tique, structurc cristalline, propri6t6s physicochimiques. 554 THE CANADIAN MINERALOGIST

INTRoDUcrtoN

According to the stability diagramsof Alwan & Williams (1980),the two uranyl carbonateminerals, liebigite,Ca2[UO2(CO3)2]'10H2O, and andersonite, CaNa2[UO2(CO3)2]'6H2O,should be expectedto co- exist, Severalsyntheses of theseminerals have been described(Blinkhoff 1906,Axelrod et al. 1951, Bacheletet aL \952, Meyrowitz& Ross 1961, Meyrowitzet al.1963, dejka 1969,Coda et at. 1981). Meyrowitz et al. (L963) mentionedthe formation of unknown yellow-green crystalsobtained during the synthesisof liebigite from potassiumcarbonate, uranyl nitrate and calcium nitrate. In a previouspaper, Vochten et al. (1993) described a new methodfor the synthesisof liebigite and ander- sonite starting from tetrasodium uranyl carbonate Naa[UO2(CO:)z](NaUTC). As mentionedin that paper,the formation of an intermediatephase, consist- ing of well-formed yellow-green lath-like crystals, was observed.The X-ray powder-diffractionpattem of this phasecould not be matchedwith that of any known compound. For the presentsynthesis, two grams of NaUTC were dissolvedin 100 mL of 0.04 M CaCl2and left to evaporatein an openvessel at 25oC.After one week, well-formed hemisphericalcrystals of liebigite started to grow. After two weeks,the formation of crystalsof the intermediatephase was observed,and after three weeks, crystals of andersonitewere formed. The aim of the presentstudy was to determinethe chemical compositionand the crystal structureof the unknown syntheticphase, as well as someof its physicochemi- cal characteristics.

CRYSTAL MoRPHoLocY

The scanningelectron micrographs (Fig. 1) show that the crystals are orthorhombic,with a lath-like habit and facetededges. The length of the crystals rangesfrom 0.6 to 1.0mm, andtheir width, from 0.01 to 0.1 mm. As cleady shownin the SEM photographs, the crystalsare built up of subparallellayers. Frc. l. Scanningelectron micrographs of theintermediate The morphology of the crystals can be considered phasebetween liebigite and andersonite. Scale bar: A' as a combinationof the forms {010} and {001}. The B: 100pm; C: 10Pm. two othervisible forms, {01I } and {0Tl }, are barely developed.The layering in the photographsis parallel to (001). TGA 951 apparatuswas used, with an applied rate of heatingof S'C/min and a constantflow of N2 of CmurcatCowosmor 30 ml/min. In orderto detectthe temperatureat which CO2 is liberated, the outlet of the gas streamwas The Na, Ca and U content was determinedby passedthrough a Ba(OH)2solution. The resultsare electron-microprobeanalysis. Oligoclase (Na), wollas- shown in Figure 2. The thermal decompositionof the tonite (Ca) and UO2(U) were usedas standards.Data carbonategroups starts at approximately300oC and is were collected at 12 different points. The water and completeby 800'C, with a loss of 2lVo by weight, carbon dioxide contentwas determinedby thermo- correspondingto three carbonategroups. It is clear gravimetry (TG) in combination with differential from the TG curve that the dehydrationand decar- scanningcalorimetry @SC). A DupontDSC 910 and bonation stepsoverlap, so that it is quite difficult to SYNT}TETICPIIASE BETWEENLIEBIGITE AND ANDERSONTIE 555

Omcar,PanarcrBns

The crystals are biaxial negative,with 2V = 65' (t5'), and have a positive elongationand parallel extinction. The indices of refraction are s.1.525 (t0.005), P 1.s47(10.002) and 1 1.556(10.002). The indices p and y were determinedby meansof liquids with known index of refraction. The value of cr was calculatedfrom B, y and 2V. Most of the crystals are clearly twinned, as can be detectedby polarizing microscopy.

LulanrcscrNcnSpecrRA

Both fluorescenceand phosphorescencespectra were recordedby meansof a Perkin-Elmer MPS44B specuofluorimeterat298 and77 K, with an excitation wavelengthof 380 nm, a bandpassfor excitationof 20 nm and for emissionof 5 nm, and a scanspeed of 120nm/min. 2ao 4oo 8oo looo r*p1*mfq Frc. 2. Resultsof thermogravimetric(TG) anddifferential scanningcalorimetry @SC) analyses of theintermediate phasebetween liebigite and andersonite. z a

determine C the temperatureintervals over which the o individual molecules of water are lost. Only the total o amountof watercould be estimated(l5.4%o); it corre- c spondsto 5.38 water molecules.Table I summarizes -a the compositionin terms of oxides,from which the chemicalformula is calculatedby the classic"residual oxygen"method, based on I I oxygenatoms. This leadsto the formula: Ca1.raNan.63[UO2(COr)3]..xH2O (-t= 5.38). Becausethe electroneutralityrule is to be obeyed, the formula lacks 0.29 positive charges.This problem can be solved by the presenceof a proton, associated with an oxygenatom. This could be an oxygenof UO, or an oxygenofH2O, as hasbeen suggested for other uranium minerals by Smith et al. (1957), Brasseur (1962) and Sobry & Rinne (1973). In this case,the formula can tlen be written as: Ca1.r4Nq.6:(H:O)o.zs [uo2(co3)3]-5H2o.

TABLE 1. CI{EMICAL OOTIPOSITIOI\ OF TI{E INTERMEDIATE COMPOUND

oxide weigbr% quantities ratio

Na2O 3'.12 0.0503 0.63 Emlsslon(nm) u03 47i22 0.1651 1.04 CAZ 2L00 0.472r 3.00 Frc. 3. Luminescencespectra of the intermediatephase HzO t5.36 0.8533 5.38 betweenliebigite and andersonite. Total 100.46 A. Fluorescencespectra at77 and298K. B. PhosphorescencespectraatTT K. 556 T}IE CANADIAN MINERAIOGIST

TAII.B 2 REI,IITYE INTENSIIIESOF TgP PHOSPHORESCEN(EBANDS AT 7r K width 1.05' in 35 stepsplus cr1-cr,dispersion, rcR LIEBIoTTETllE S!'T{THS[CINTERMEDIAIE @MFOTJND, AND ANDERSONITE measuringtime/step between 0.5 and 2 s) was usedto collect 12 632 reflectionswith indices-2 < h <26, -2 < k < 24,-2 < I 3 26 and20^u = 60o. Of thesemeasured reflections, 12 602 wereaccepted, htermedistecompound 8.0 wereconsidered independent reflections. Of Andemnite 3.5 and9 014 the 9 014 uniquereflections (R6, = 0.018),4 861 with I )2o. [o(I) from counting statistics]were considered obserVed(Stoe 1988a).The standarddeviation in intensity was computedfrom o21I1= (P + k2B + The fluorescencespectra of the intermediatephase 0.0002P2), where P standsfor peakcounts, k is back- at 298 and77 K are shown in Figure 3. The intensity ground to peak-scantime, and B is total background of fluorescenceat 298 K is much lower than that at count. The intensities of three standardreflections 77 K, which can be explained by the fact that the (400, 040, 004) measuredevery hour showedan aver- quantumefficiency of fluorescencedecreases with agedecay of 7.4Voover the courseof the datacollec- increasingtemperature. The band-gapenergy is calcu- tion. The intensitieswere correctedfor Lorentz and lated at the most intensepeak (505 nm) as 2.46 eV, polarizationeffects. Empirical absorption-corrections which indicatesthat the compoundstudied must be basedon v scans(North er al. 1968,Stoe 1988c) were regardedas an insulator, in the sameway as liebigite applied using nine reflections rraa;tf = 90' in the and andersonite(Vochten et al. 1993). range8 <20 <36. Reflectionswere measured at 10" At 298 K, the intermediatephase shows no phos- intervalsfrom 0 to 360'. The minimum and maximum phorescence,as is also the casefor liebigite and ander- transmissionvalues ranged from 0.021to 0.068. sonite(Vochten et al. 1993).At 77 K, however,a Systematicabsences of Oftlfor k + I = 2n + 1 atd well-pronouncedphosphorescence could be detected h\l for h + I = 2n + l, as derived from Weissenberg for all threecompounds. The phosphorescencespec- photographsand a computeranalysis ofthe measured trum ofthe intermediatephase at 77 K (Fig. 3) canbe data with the programSYSABS (Gabeet al. 1989), characterizedby six bands(485, 505, 523,550,574 indicatedPnnm or Pnn2 as the spacegroup. The and 600 nm). In Table 2, the intensitiesof the indi- centrosymmetricspace-group Pnnm was favoredby vidual phosphorescencebands are given, togetherwith intensity statistics.The structurewas successfully thoseofliebigite andandersonite. From this table,it is solved in this spacegroup using the VAX version of clear that the intensity of phosphorescenceis strongly the DIRDIF system(Beurskens er al. 1992).The auto- deoendenton the CalNa ratio. A partial substitutionof matic run located 36 atoms other than hydrogen.All C** Ay Na+ions is reflectedin ivery strongdecrease other atoms were found from subsequentdifference- in the intensity of phosphorescence. Fourier maps. The structurewas further refined with the PC version of the NRCVAX crystallographic Cnvsrnl- Srr.ucnts softwarepackage (Gabe et al. 1989).Anisotropic least-squaresrefinement of F convergedat R = 0.0475, The crystal chosenfor data collection, a clear R* = 0.0562.The atomsof hydrogenwere not lo^cated. yellow-green lath of dimensions0.07 x 0.11 x The function minimized was lw(lF"l - lF"l)zwith 0.76 mm, was mountedwith the long dimensionalong w = ll(&(F) + 0.00030F"2), with ofo) from count- the fiber. It exhibits a well-pronouncedmorphology ing statistics.The final refinementafforded values of and is not twinned when viewed in a polarizing micro- S (goodnessof fit) equaln 1.67and (A/o).* lessthan scope.The experimentaldensity was determinedto be 0.01.The crystalstructure has also been refined in the 2.88 Mg/m3by flotationin a mixture of 1,1,2,2tetra- noncentrosymmetricspace-group Pnn2, tesultrngin a bromoethaneand n-hexane.Data were collected at slighlyhigher rt value(4.79). Based on a R-factorratio room temperatureon a Stoe STADI4 single-crystal test(Hamilton 1965), we cannotreject atthe 5Volevel diffiactometerequipped with MoKo, radiationand a of significancethe hypothesisthat Pnnm is the correct perpendicularmounted graphite (002) monochro- mator. Ascurateunit-cell parameterswere obtainedby least-squaresrefinement of measuredand calculated (sin0)2values (24 reflections).The O-valuesare free of TABLE 3. TIIB IMPORTANT CEOMEIRIO{L DATA SX{GRNINO any zero-point error of the circles. They are obtained THB CATTON-OXYCENBONDS IN UOdOOi\ by measuringthe o-centers of reflections at positive Av@ge At@ge Av@ge and negative20 and taking the difference of the two u€op. u-o.q. Uouq. c€"c. c4- cof- ovvaluesas tle true 20 value(Stoe 1988b). This leads Andersonite 1.19 2.41...2.6 2.M 1.26...1.311.28 1.30 Liebigite 1.78 2.39...2.45 2.43 1.28...r.31r.29 t.5 to the following parameters:a-18.150(3), b 16.866(6),, ' Pcmt L7B(l\ 2.4t...2.47 2.4412' 1.2s...t.32t.3O\2) t.X(2) c 18.436(3)4,, v seqq?) A3, D""r"= 2.84 Mglm3, cmDomd F = 11.5 mm-t, Z = 8. The co-scantechnique (scan ap, mem apical eq, mro equtorial md te. mm tsEital SYNTHETICPIIASE BETWEENLIEBIGITE AND ANDERSOMIE

5 3 s€ -s !r ^

-'6m=

q E.: o ts 5€;"8g6.E t +r o E d . o5 d s F 558 TIIE CANADIAN MINERALOGIST space-group.A similar analysisof the final difference with the uranyl-bondedoxygen atomsinvariably is map revealedmaximum and minimum oeakscorre- smaller(between 112.7 and 116.9")than the other spondingto 3.6 eA-3and -1.53 eA-3.Atomic scatter- ones(between 119.4 and 124.6").The averagebond- ing factors were taken from International Tables for length for the oxygen atoms coordinatedto U is X-ray Crystallography(1974, vol.IV), andanomalous greiter (mean value: L.299 A-1ttran the terminal C-O dispersionwas appliedfor eachtype of atom.General bonds(mean vafue 1.249A), suggestinga higher calculationswere performedusing the PC version p characterfor the latter. Table 3 summarizesthe of the NRCVAX package,PARST (Nardelli 1983), importantgeometrical data for the oxygenbonds in PLATON (Spek 1990)and PLUToN (Spek 1992). uo2(co3)3. Crystallographicdata are summarizedin Table 3. Two (Ca6,Ca7) of the four Ca polyhedralie on the Inspectionof the final difference-Fouriermap reveals mirror plane. One of them (Ca6) is six-coordinated a positivepeak at x = 0.000,/ = 0.500,z 7 0.034, [CaOa(H2O)2].One pair of symmetry-relatedoxygen correspondingto the maximumpeak of 3.6 eA-3in the atoms(Ol 13)occupies the lrst two coordinationsites. difference map mentionedearlier. This peak contains Two other atomsof oxygen (0322 and 0222) occupy approximatelyhalf the numberof electronsof an the next two coordination sites. Another pair of oxygenatom and is locatedat a reasonableH-bonding symmetry-relatedwater molecules(061) occupythe distance(2.87 A) from the oxygen atom of another positionsfive andsix. Thesewater molecules are at an watermolecule. Voids calculatedin the residual appropriatedistance from Ol13 and O13l for hydro- structurewith the programPLATON show a cavity of genbonding. The Ca7atom lies in the symmetryplane 54 A3 at the positionof the highestdifference-Fourier and is sifuatedin a [7]-coordinatedenvironment, coor- peak,which indeedcan be occupiedby a watermole- dinated by a symmetry-relatedpair of carbonateoxy. cule,which normallyoccupies some 40 A3. This posi gen atoms(o31.2), two pairs of symmetry-related tion is very nearthe planeof symmetry,which means watermolecules (O7l andO72), and onewater mole- that the two symmetry-equivalentpositions are too cule (O73) lying in the planeof symmetry.Both O7l closeto eachother. This situationis acceptableonly if and O73 belong also to the Na9 sphereof coordina- the electrondensity is distributedover two different tion. Atom O71 alsomakes hydrogen bonds with other locations,with a site occupancyof 0.5. This is a case water moleculesO8l and O92. and atom O73 with of statisticaldisorder: one has to be careful in search- O82.Hydrogen bonds exist also amongO72, Ol2 and ing for small peaksin electrondensity in a Fourier 0211. The Ca4 atom sharesedges with two uranyl map dominatedby U atoms.Because of the small polyhedra(0112, 0122, O3Ll and O321)and is numberof electrons,which gives it more the statusof coordinatedto two terminalcarbonate (0213, 0133) a residualpeak, we did not include it in the refine- oxygenatoms and one water molecule (O41). The last ment. Ca atom(Ca5) is alsoseven-coordinated, shares edges Lists of structurefactors and anisotropicthermal with two uranylpolyhedra(0221 andO2l2, 0132 and parametersare available from the Depository of 0111), and is alsocoordinated to two terminalcar- UnpublishedData, CISTI, NationalResearch Council bonateoxygen atoms (0123, 0313) and one water of Canada,Ottawa, Ontario K1A 0S2. molecule(O51). The structureand numberingscheme of atomsin The Na8 and Na9 atomsare both l5]-coordinated, CarNar.r(HrO)o.s[QOz)z(CO:)o].8HzO (M, = 1208.47) but the compositionof the nryopolyhedra is different. are illustratedin the PLUTON plot (Spek 1992), The Na8 polyhedracan be describedby NaO2(H2O)3, shownin Figure 4. The basicstructural unit consistsof with oneof the watermolecules (O4l) at a ratherlarge three UO,(CO,), units. Two of them (U, and Ur) distanceof 2.68A. The otherwater molecule (O81) is lie on a plane of mirror symmetry.Each U atom is involved in hydrogenbonds with tle oxygen atoms [8]-coordinated.The resultinghexagonal-bipyramidal Ol2l, O3l3 and O7l. The Na9 atomlies in the plane polyhedronis distorted.The two apical uranyl oxygen of symmetryand is surroundedby four H2Omolecules atomsare much closerthan the oxygenatoms from the and one apical oxygen atom 031, to give bidentateCO3 groups in the equatorialcoordination. [NaO(HrO)o].The meanNa-O distancelies between The UO22*ion is alwaysnearly linear [averageO-U-O 2.44 and2.66 A. The Ca-O and Na-O bondingdis- angle 178.8(7)'l and has a meanbond-length of tancesto water moleculesare generallythe longest 1.78(1)A. onesin the Ca as well as in the Na spheresof coor- The U-O carbonatebonds vary between2.420(9) dination. and 2.468(8)for Ul, between2.413(9) and2.434(9) The structurecontains water moleculesin nine for U2, and between2.417(9) and 2.459(10) for U3. positions;two of them are specialpositions in the The C-O bondlengths vary from 1.242(3)to 1.321(2), planeof symmetry. correspondingto the range reportedfor liebigite Chargebalance is achievedby OH- substitutionfor (Mereiter 1982) and andersonite(Coda et al. l98l). O2-. The single-bondcharacter of a normal 02- bond The CO. groups have the sp2 configuration, with an shouldincrease, and the bond length shouldbecome averageangle of 120o,but the O-C-O angle formed larger,which has not beenobserved. Electrical neu- SYNTHETICPI1ASE BETWEEN LIEBIGITE AND ANDERSONITE 559

TABLE 4. FINAL ATOMIC CDORDINATES AND EQUIVAUNT ISOII'OPIC 20Vohigher. The analysisdoes not show the exact (41' THERMAL PARAII'E'TERS position of the positive chargein the structure.Since the O92 oxygen is the only one not belonging to the vlb ueq sphereof coordinationof the positiveions Ca2+and Ca4 0.J576(t) 0.6740(2) 0.1861(2) 0.0188(7) Na+,tle positivecharge most likely is locatedin this Ca5 0.4370(l) o.82A6Q) 0.3147(2) 0.0199(7) Ca6 0.7516(2) 0.5268(2) 0.0000(0) 0.0236(8) position. Cal 0.3196(2) 0.9258(2) 0.0000(0) 0.024(1) Figure 4 showsthat the UO2(CO3)3units form Na8 0.667s(4) 0.s292(4) 0.2u2(5) 0.054(3) layersparallel to (011).The layersare interconnected Na9 0.6787(6) 0.8640(6) 0.0000(0) o.074(5) of Ca polyhedra.The sodiumions Na8 and ul 0.7470(3) 0.7eMQ) 0.2278(2) 0.020(1) by layers ol l 0.7413(6, 0.8380(6) 0.1422(5) 0.036(3) Na9 are locatednear the Ca layers, and connectto the ot2 0.7521(5) 0.7443(6) 0.3145(5) 0.032(3) oxygenatoms of the Ca environment. cll 0.7493(8) 0.637'r(7) 0.1558(6) 0.023(3) The final atomicparameters are listed in Table 4. ol r r 0.8074(5) 0.6800{6) 0.1683(6) 0.02e(3) ol 12 0.6892(5) 0.6703(6) 0.1808(7) 0.03e(4) Tables of interatomic distancesand anglesare avail- o113 0.7518(6) 0.5721(5) 0.1241(5) 0.029(3) able from The Depositoryof UnpublishedData, c12 0.6085(7) 0.856e(8) 0.2719(8) 0.025(4) CISTI, NationalResearch Council of Canada,Ottawa, ol2l 0.6706(5) 0.8909(6) 0.2834(6) 0.032(3) or22 0.6120(5) 0.7909(6) 0.2368(6) 0.03r(3) OntarioKlA 0S2. ol23 0.5484(5) 0.8878(6) 0:931(7) 0.038(4) cl3 0.3853(7) 0.6383(? 0.2372(9) 0.026(4) X-Rav Poworn-DnmacrroN Dara ol3l 0.3229(5) 0.6009(5) 0.2276(6) 0.028(3) ot32 0.3826(5) 0.7030(6) 0.2703(6) 0.028(3) ol33 0.4435(5) 0.6108(6) 0.2091(6) 0.032(3) X-ray powder-diffractiondata were recordedat u2 0.st22(4) 0.7210(4) 0.s000(0) 0.020(2) 40 kV and20 mA, usingCuKal radiation(1, = 1.5406 o2r 0.5911(7) 0.7820(9) 0.5000(0) 0.03s(5) A). The diffraction pattem is recordedby meansof a on 0.4334(7) 0.6571(9) 0.5000(0) 0.033(5) c2r 0.5598(7) 0.657\8) 0.3635(8) 0.024(4) Guinier-Hiigg camerawith a diameterof 100 mm. 0211 0.5780(5) 0.072(6) 0.42s4(6) 0.034(3) Silicon powder (NBS-640) was used as an internal o2l2 0.5158(5) 0.7176(6) 0.368(5) 0.030(3) standard.The relative intensitiesof the diffraction o2r3 0.5828(6) 0.6300(6) 0.3045(s) 0.032(3) c?2 0.411(1) 0.855(1) 0.5000(0) 0.030(7) lines were measuredwith a Carl ZeissJena MD-100 ozzt 0.4394(5) 0.8219(6) 0.4415(6) 0.035(4) microdensitometer.Using the previouslydetermined 0222 0.3636(8) 0.e07(l) 0.5000(0) 0.043(6) unit-cell parametersand spacegroup, the powder u3 0.4792(4) 0.763(4) 0.0000(0) 0.01e(2) o3r 0.s465(7) 0.8372(9) 0.0000(0) 0.036(6) pattemswere indexedby the program PPLP from the o32 0.4t22(9) 0.6852(9) 0.0000(0) 0.040(5) IBM PC versionof NRCVAX (Gabeet al. 1989). c3l 0.4255(7) 0.858(8) 0.136e(7) 0.022(4) Nearly all the observedreflections could be indexed, o3l1 0.4743(5' 0.769(5) 0.1330(6) o.a7G) (A given o3L2 0.39815) 0.8465(6) 0.0742(5') 0.028(3) with AQoo,3 AQ"ur" < 0.05").The resultsare o3l3 0.4084(5) 0.859e(6) 0.1939(6) 0.030(3) in Table5. c32 0.594(1) 0.643(l) 0.0000(0) 0.038(8) o32r 0.5653(6) 0.6746(6) 0.0590(6) 0.037(4) Col|DArmnrrv 0322 0.6371(8) 0.s861e) 0.0000(0) 0.03e(6) o4r 0.5885(6) 0.5372(6) 0.1616(7) 0.045(4) o51 0.4115(6) 0.960,1(5) 0.3423(6) 0.034(4) The compatibilityindex | - (KJK) is calculated 061 0.698(l) a.42r8(t) 4.W47(7) 0.088(7) using the Gladstone- Dale constants(Mandarino o71 0.8640(6) 0.4719(6) 0.4171(6) 0.041(4) o72 0.7299(6) 0.626(e) 0.4181(6) 0.058(5) 1981),the meanindex of refractionand the density. o73 0.n6rQ) 0.4682(8) 0.5000(0) 0.037(4) For this index, a value of -0.004 is obtained,which o81 0.2956(6) 0.97r2Q\ 0.2W4t6) 0.041(4) indicates"superior" consistency of the data obtained o82 0.6137(6) 0.4667|0 03906(? 0.053(4) o91 0.260(l) 0.747(L) 0.5000(0) 0.06e(8) for the compound. ov2 0.500(0) 1.000(0) 0.1561(e) 0.043(5) Drscussrox

u,=4\,l,u,{d,a,.a, During the synthesis,starting from NaUTC and Ca2*ions, liebigite, the intermediatephase and ander- . b p8eoth68 E$hoad dddad d{iatioo it thom sonite are obtainedconsecutively. Therefore, the intermediatephase must be consideredas a stable phase.The sequenceof crystallizationof the tlree trality can alsobe preservedif watermolecules occur compoundsis clearly govemedby the Ca2+/1.{a+ratio. in the form of oxoniumions, as was suggestedby It is also clearthat the intensityof the luminescence Smith er al. (1957),Brasseur (1962) and,Sobry & specffais stronglydependent on this Ca2+ArIa+ratio. Rinne (1973).Bond-valence analysis (Brown & Wu For this phase,a deficiencyin positive charge 1976,Brown 1976)of all oxygenatoms that are part seemsto exist. This deficiencycan be derivedfrom of a water moleculeindicates that all bond valences the structuredetermination as well as from the chemi- arewithin 10Voof the expectedvalue except for O51, cal composition.In agreementwith Brasseur(1962) which is 207olower, and O71 and O73, which are and Sobry & Rinne (1973),we proposethe presence 560

TABLE 5. X-RAY POWDER-DIFFRACNONDATA meters. We are also indebted to Mr. Wauthier of the FOR THE INTERMEDIATESYNTHETIC PHASE Department of Geosciencesof the University of Louvain-la-Neuve for the electron-microprobe analy- d(obs) Uls(obs) d(calc) hkl sis" and to Mr. J. Cilis of the Museum of Natural History of Belgium for making the scanning electron micrographs. The valuable comments of Drs. D.K. 12.95 101 t2.44 0ll Smith, Jr. and J. Szymafski were very helpful in 10.20 l0 to.27 1ll refining the article. 9.07 t0 9.22 002 8.580 90 8.434 020 RBrsnnwcss I.JJZ aL I 6.510 70 6.467 202 6.198 l0 6.180 220 AlwaN, A.K. & Wnr-iavs, P.A. (1980):The aqueouschem- 6"045 212 istry of uranium minerals.2. Minerals of the liebigite 5.850 20 5.886 t22 s.771 013 grotp.Mineral. Mag. 43,665-667. s.isr 30 5.500 113 s.420 l0 5.447 3ll A:cnoo, J.M., Gmaar-or,F.S., MrlroN, C. & Mrnera, K.J. 5.328 4 5.381 031 (1951): The uranium mineralsfrom the Hillside mine, 5.089 100 5.133 222 YavapaiCounty, Arizona. Am. Mineral. 36, 1-22. 4.892 30 4.9t8 320 4.605 50 4.609 004 4.539 4A 4sU 400 BacHnnr, M., Cnrvlex, E., Dours, M. & GowerrE, J.C. 4.381 8 4.388 410 (1952):h6paration et propri6t6sdes uranylcarbonates. 2. 4.32r 8 4.34t 322 Uranylcarbonatesalcalino-terretx. Bull. Soc. Chim. 4.230 8 4.2t9 040 France19, sdr. 5,565-569. 4.096 8 4.t49 033 4.lll 140 4.019 50 4.048 024 BEURSKINS,T.P., Aovrnael, G., BEURSKENS,G., BosMAN, 3.969 50 4.001 420 W.P., Gencra-Gnaxoe,S., Got[D, R.O., Swrs, J.M.M. 3.952 t24 & Swrerla, C. (1992): The DIRDIF program system. 3.847 l0 3.837 042 Tech.Rep., CrystallographyLoboratory, Universityof 3.7M 30 3.757 142 Nijmegen, Nijme gen, TheNetherlands. 3.691 l0 3.696 224 3.527 70 3.536 242 3.469 40 3.463 340 BLINKoFF,C. (1906):Beitrage zur Kenntniskondensiener 3.414 J-+2I Uranyl-verbindungen.Phil. Diss., Univ. Bern, Bern, 3.360 5 3.351 215 Switzerland. 3.325 5 3.321 051 3.28s 432 BRAssEUR,H. (1962): Essai de reprdsentationdes oxydes 3.24r 40 3.236 404 3.115 40 3.114 044 doublesmX++O.UO3.nH2O par une formule gdn9rale. 3.085 4Q 3.093 440 Bull. So c. F r. M indral. Cristallo gr. 85, 242-244. 3.070 40 3.070 r 44 3.427 40 3.021 424 BRowN,I.D. (1976):On the geometryof G-H...O hyclrogen 2.9t13 50 2.9116 206 bonds.Acta Crystallogr.A^32, 24-31. 2.8786 50 2.87s0 602 2.8532 20 2.8518 126 Wu, KaNc-KuN(1976): Empirical paramete$ 2.8160 30 2.8133 253 & 2.7561 50 2.75t0 226 for calculatingcation-oxygen bond valences.Acta Cry stallo gr. 832, 1957-1959. + additional lines. Complete list available on request. CrJKA,J. (1969):To the chemistryof andersoniteand ther- mal compositionof dioxo-tricarbonatouranates.Coll. Czech.Chem. Commun. 34, 1635-1656. of oxonium ions in the structurein order to obtain Cooa, A., DELLAcrusrA, A. &Tezzorr, V. (1981):The electroneutrality. structureof syntheticandersonite, Na2Ca[U02(C03)3] .xH2O(x = 5.6).Acta Crystallogr.837,1496-1500. Specimensin mineralogicalcollections in which liebigite and andersoniteoccur togethershould be re- GABE,E.J., LrPacs, Y., CuenlaNo, J.-P., LEE,F.L. & examined,as the intermediatephase may be present. WurrE,P.S. (1989):NRCVAX - an interactivepro$am systemfor structureanalysis. J. Appl. Crystallogr.22, AcrxowtnpceMENrs 384. (1965): The authorsthank the National Fund for Scientific HAMrLroN,W.C. Significancetests on tlte crystallo- graphicR factor.Acta Crystallogr.18, 502-5 10. Research(NFWO) for financial support,awarded to R.V. We thank Dr. F. Mees of the Laboratoryof MaNocRD.ro,J.A. (1981):The Gladstone- Dale relationship. Mineralogy,Petrography and Micropedologyof the IV. The compatibility conceptand its applications.Can. University of Ghent for determiningthe optical para- Mineral.19.441-450. SYNTHETIC PHASEBETWEEN LIEBIGITE AND ANDERSONTIE 56r

Mnnurrrn, K. (1982):The crystal sffucrureof liebigite, SIEK,A.L. (1990): PLATON - an integratedtool for the CarUO2(CO3)3.-77H2O. Ts c he rmal

MBynowrrz,R. & Ross,D.R. (1961):The synthesisof large -(1992): PLUTON - a VersatileMolecular Graphics crystals of andersonite.U.S. Geol. Surv., prof. pap. Iool. (version Jan. 1993). Bijvoet Center for 42/.8.266. BiomolecularResearch, University of Utrecht, The Netherlands. & Wrsrs, A.D. (1963):Synthesis of liebigite.U.S. Geol. Surv., Prof. Pap. 4758,162-163. SroB & Co. (1988a):REDU4. Data ReductionProgram (version6.2). Stoe & Co.,Dannstadt, Germany. NARDELLT,M. (1983):PARST: a sysremof FORTRANrou- tines for calculatingmolecular structureparameterc from (1988b):DIF4. Dffiactometer Control Program resultsof ctystal structureanalysis. Comput. Chem. 7, (version6.2). Stoe & Co, Darmstadt,Germany. 95-98. (1988c):EMPIR. Empirical Absorption Correction NoRrn, A.C.T., I,nnups, D.C. & Marurws, F.S. (196g):A Program(version 1.2). Stoe & Co.,Darmstadt, Germany. semi-empiricalmethod of absorptioncorrection. Act4 Crystallogr. AU, 351-359. VoCHTEN,R., VaN HAVERBEKE,L. & VAN SpnrNcEr-,K. (1993):Synthesis of liebigiteand andersonite,and study Sumr, D.K., Jn., Gnusn, J.W. & LFscoMr, W.N. (1957): of their thermalbehavior and luminescence.Can. The crystal structureof uranophane[Ca(H3O)r] Mineral.3l. 167-171. (UOr2(SiOr2.3HrO.Am. Mineral. 42, 59+618.

SoBRy,R. & Rttl\E, M. (1973):A least-squaresmethod for the identification of resonantgroups from NMR powder spectra.Identification of oxonium and hydroxyl ions in Received July 16, 1993, revised manuscript accepted hydrateduranates. J. Nucl. Res.12,152-167. January6,1994.